A large amount of plastic foams have been used as packing and packaging materials based on lightweight properties, cushioning property and formability thereof, and are made of raw materials, for example, chemical products from petroleum, such as polystyrene (PS), polyolefin and the like. However, it is difficult to effect disposal of the plastic foams after use to cause the following serious social problems. That is, a burning furnace is damaged by high combustion calorie generated on incineration and, furthermore, the plastic foams are not degraded when buried under the ground and also occupy the space of the place for disposal due to large volume.
A harmful influence of the untreated waste foams to be exerted on the natural ecosystem, for example, contamination of rivers, seas, etc. has becoming more serious. Therefore, there has been developed a biodegradable resin, which is degraded in the ecosystem and exerts little influence on the global environment. For example, a polyhydroxybutyrate resin synthesized in the microorganism, or a polyester of an aliphatic glycol and an aliphatic carboxylic acid, or a polyester resin containing caprolactone as a principal component has been suggested. However, the former is inferior in purity because it is synthesized by microorganism, and is also drastically inferior in productivity and use thereof is limited.
The latter is surely superior in productivity because it is prepared from those, which are easily available in large amount at cheap price, such as petroleum natural gas. However, it is not suited for practical use as a biodegradable foamed resin because it is a crystalline resin and has a low grass transition temperature. Furthermore, since it is prepared from the petroleum natural gas and a carbon dioxide gas is newly added to a carbon dioxide gas system that is present on the surface of the globe when it is degraded. Therefore, it does not contribute to the inhibition of an increase in carbon dioxide gas. From a long-term point of view, the availability is lowered in future because its raw material source is limited, thereby making it impossible to substantially contribute to the protection of the global environment.
Furthermore, a polymer of glycolic or lactic acid as a biodegradable raw material has been obtained by ring-opening polymerization of glycolide or lactide and employed as medical fibers. However, since the crystallizability as a constituent feature for fiber-forming capability is imparted to a resin, the resin polymer has never been used widely as a packaging container or a cushioning material if it is not formed into a foam.
An object of the present invention is to provide an expandable resin composition which has not only biodegradability but also excellent productivity, i.e. an expandable resin composition which is degradable due to microorganism and causes less burden to the global environment on disposal after use, and which has high productivity and is suited for practical use.
To attain the object, the present inventors have studied intensively about essential conditions for a biodegradable resin having high expandability with respect to base polymers, additives for increasing the molecular weight, and additives used for foaming. As a result, they have found a biodegradable resin composition having enough productivity to be suited for practical use, thus completing the present invention.